The present invention relates generally to devices and systems for controlling and regulating the conversion of power from an AC source to a load, typically a DC motor. More particularly, the present invention relates to devices which control the conduction of controllable rectifier, e.g., thyristor, bridges placed between the source and the motor and, in particular, methods for compensating for inaccuracies caused by line voltage imbalances during power conversion.
Motor control systems of the type described above typically include at least one rectifier bridge connecting the motor windings to alternating voltage supply lines. For a conventional three phase motor, each AC phase line is generally coupled to the motor by a pair of connected thyristors. That is, in a three phase system, six thyristors are required to transfer power from the source to the load, each for one half of each phase. A thyristor, such as a silicon controlled rectifier (SCR), is generally defined as a switchable diode controlled by a gate element. Each thyristor presents a relatively high blocking impedance to the flow of electrical energy until it is forward biased by a trigger current being applied to its gate element. A digital control circuit typically determines the proper time to trigger the thryistors during each half-cycle of the supply line voltage. Once a thyristor is triggered by the application of a predetermined current to its gate, the forward blocking impedance is lowered, thereby permitting the flow of electrical energy through the thyristor in the manner of a diode. Once conduction has been enabled, the thyristor cannot be turned off until the current flowing therethrough is reduced to near zero (i.e., makes a zero crossing).
The amount of power transferred to the motor is controlled by varying the duration of the conduction of the various thyristors. This is done by controlling the firing angle of each thyristor, that is, the point in the AC waveform at which the thyristor is initiated into conduction. The process of switching from thyristor to thyristor is known as commutation. In order to control the firing of the various thyristors, conventional systems incorporate a firing controller, either analog or digital, to control the firing angle of each thyristor.
Although input voltages received by the power converting system are preferably stable and consistent, external disturbances such as an imbalance in the grid voltage introduce undesirable fluctuations in the input voltages. A grid voltage imbalance can be caused by a number of factors such as supply transfer malfunction, neighboring unbalanced loads, and other grid problems. Continued operation of the power conversion system during such circumstances may have several deleterious effects like overloading of the individual phase legs and various additional system components.
Conventional thyristor based power converter applications utilize either preprogrammed values or a single line to line voltage measurement in controlling the firing of the various thyristors. Unfortunately, these methods of firing control permit the introduction of inaccuracies due to line voltage imbalances of the type described above.
Accordingly, there is a need in the art of power control systems for a system and method for accurately compensating for the effects of line imbalances, thereby reducing or preventing the above-described deleterious effects.
The present invention overcomes the problems noted above, and provides additional advantages, by providing a method and system for compensating for line imbalances in line commutated converters. A bridge firing controller receives signals representative of two of the line to line voltages received by the bridge. The controller includes a phase-locked loop (PLL) synchronizing tool which receives the line to line voltage signals and generates firing angle and frequency signals used for synchronizing the two input signals. The PLL controller, in addition to the firing angle and frequency signals, also generates signals representative of filtered values of amplitudes for each of the line to line voltage signals. The PLL controller further incorporates a voltage imbalance compensation algorithm for receiving the filtered amplitude signals and also a signal representative of the firing sector of the bridge. In operation, the voltage imbalance compensation algorithm generates, based upon the sector signal and the received filtered amplitude signals a amplitude signal used by a current regulator which is compensated for line imbalances present in the line voltage.